Photosensitive flexographic printing plates are becoming more important in the printing trade. Particularly useful compositions for preparing such flexographic printing plates include: compositions containing addition-polymerizable, ethylenically unsaturated compounds, photoinitiators and as polymeric binders either block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene polymers, or butadiene/acrylonitrile polymers with optional carboxyl groups. The printing plates utilizing compositions containing said block copolymers are generally compatible only with alcohol or alcohol-acetate based inks; while printing plates utilizing compositions containing butadiene/acrylonitrile polymers with optional carboxyl groups are generally only compatible with hydrocarbon based inks.
The flexographic printing plates containing styrene-butadiene-styrene or styrene-isoprene-styrene block copolymer binders, after imagewise exposure and development, are generally treated with an acid hypochlorite solution to reduce the tackiness of the layer. A reduction in tackiness can also be achieved by use of a bromine solution; however, neither chlorine nor bromine treatment renders printing plates based on these block copolymers compatible with hydrocarbon based inks.
Flexographic printing plates containing a butadiene/acrylonitrile polymer with optional carboxyl groups, however, are not only tacky but are susceptible to attack by certain solvents and other components present in some printing inks, e.g., alcohols, acetates, etc. Use of these plates is limited therefore to a restricted class of inks containing substantially hydrocarbon solvents. The use of a chlorine-containing solution such as a hypochlorite solution may alleviate the tackiness problem but does not prevent the printing surface of the printing plate from being attacked by inks containing alcohols and various other solvents. Surprisingly, the butadiene/acrylonitrile polymer-containing flexographic printing plates, when treated with an aqueous solution having a free bromine concentration, either before or after postexposure, exhibit an increase in solvent resistance to all inks, and most notably in resistance to alcohol containing inks and various other nonhydrocarbon solvents. This is not observed with chlorine treatment, and it is also not observed with either chlorine or bromine treatment of styrene-isoprene-styrene or styrene-butadiene-styrene block copolymer-containing printing plates.
An object of the invention is to provide a bromination process which renders printing plates prepared from photosensitive compositions containing butadiene/acrylonitrile polymers with optional carboxyl groups compatible with alcohol-based inks as well as hydrocarbon based inks. Another object of the invention is to provide a process wherein the bromination treatment can occur prior to or subsequent to postexposure. Still another object of the invention is to provide a process for immediate bromine treatment of said printing plates which have been aqueous developed without the necessity of drying.